1. Field of the Invention
The present invention relates to magneto-optical devices, especially such as polarization controller, optical modulator, variable optical attenuator, optical isolator, optical circulator, or the like.
2. Description of Related Art
In high-speed optical communication at a transmission speed of 40 Gbps or so, polarization mode dispersion (PMD) is one essential factor of transmission failure. Recently, therefore, polarization mode dispersion compensators capable of compensating for the influence of polarization mode dispersion have become much investigated. One essential constitutive device of such a PMD compensator is a polarization controller of controlling the polarization condition of light. As disclosed in Non-Patent Document 1, lithium niobate (LN), liquid crystal, fiber squeezer, variable Faraday rotator and the like are known as the optical device to constitute a polarization controller. In particular, a polarization controller that comprises a variable Faraday rotator takes a satisfactorily short response time of a few hundreds μsec, and its structure is composed of a garnet crystal. Therefore, the optical properties such as the insertion loss and the polarization dependency loss (PDL) of the polarization controller of the type as well as the reliability thereof could be nearly on the same level as that of conventional passive modules. Accordingly, when a variable Faraday rotator is used, then a polarization controller having a better balance than any other devices can be constructed (see Non-Patent Document 1). The variable Faraday rotator comprises a magneto-optical crystal and an electromagnet that imparts an electric field to the magneto-optical crystal. This is so designed that the quantity of current to run through the electromagnet is varied so as to control the intensity of the magnetic field to be applied to the magneto-optical crystal, and the intensity of the magnetization of the magneto-optical crystal is thereby varied so as to control the Faraday rotation angle.
A method of controlling the magnetic field to be applied to the magneto-optical crystal is disclosed, for example, in Patent Document 1. The magnetic field controlling method is described with reference to FIG. 25A and FIG. 25B. FIG. 25A shows a variable optical attenuator, and the variable optical attenuator comprises a Faraday rotator (magneto-optical crystal) 113 and a polarizer 112. In addition, the variable optical attenuator further comprises a permanent magnet 114 and an electromagnet 115 that impart a magnetic field to the Faraday rotator 113 in two directions perpendicular to each other, and a variable current source 116 to give a driving current to the electromagnet 115.
The direction of the magnetic field applied to the Faraday rotator 113 by the permanent magnet 114 is parallel to the direction in which the light beam 117 runs through the Faraday rotator 113, and the direction of the magnetic field applied to the Faraday rotator 113 by the electromagnet 115 is perpendicular to the direction of the magnetic filed given by the permanent magnet 114 and to the running direction of the light beam 117.
In FIG. 25B, the arrows 102 and 105 are vectors each indicating the direction and the intensity of the magnetization inside the Faraday rotator 113; the arrows 101 and 103 are vectors each indicating the direction and the intensity of the external magnetic field; the arrow 104 is a vector indicating the direction and the intensity of the synthetic magnetic field of the two magnetic fields indicated by the arrows 101 and 103. In the drawing, the direction Z indicates the light propagation direction in the Faraday rotator 113, and the direction X is perpendicular to the direction Z. The Faraday rotator 113 is in the condition of saturation magnetization 102 owing to the Z-direction magnetic field 101 given by the external permanent magnet 114. Next, when the X-direction magnetic field 103 is given to it by the electromagnet 115, then the external magnetic field forms the synthetic magnetic field 104, and the Faraday rotator 113 is in the condition of magnetization 105. The intensity of the magnetization 105 is the same as that of the saturation magnetization 102, and therefore the Faraday rotator 113 is in the condition of saturation magnetization.
As in the above, the Z-direction magnetic field 101 is previously given to the Faraday rotator 113 by the permanent magnet 114 so that the Faraday rotator 113 is in the condition of saturation magnetization, and further the X-direction magnetic field 103 is given to the Faraday 113 by the electromagnet 115. With that, the magnetization direction of the Faraday rotator 113 is rotated by the degree θ from the magnetization 102 to the magnetization 105 by the synthetic magnetic field 104 of the two magnetic fields 101 and 103, and the intensity of the magnetization component 106 in the Z-direction is thereby controlled. Depending on the intensity of the magnetization component 106, the Faraday rotation angle varies. In the case of this method, the Faraday rotator 113 is driven all the time in the saturation magnetization region, and therefore the method is characterized in that the Faraday rotation angle can be varied with good reproducibility with no hysteresis.
However, according to the magnetic field application method described in Patent Document 1, the magnetization is uniformly rotated while the magnetic field 101 is given to the system by the permanent magnet 114, and therefore the magnetic field 103 to be given thereto by the electromagnet 115 must be strong. Accordingly, the electromagnet 115 must be large-sized, or a large current must be applied to the electromagnet 115. This produces a problem in that the size of the magneto-optical device of the type is difficult to reduce and the power to be consumed by structure is also difficult to reduce.
The Faraday rotator 113 is formed of a magnetic garnet single-crystal film that is grown in a method of liquid-phase epitaxial (LPE) growth. Light transmission through a magnetic garnet single-crystal film in the direction along the growing face of the film lowers the characteristics such as the extinction ratio of the film. Therefore, in general, light is applied to the film in the direction perpendicular to the growing face of the film. In this case, however, the thickness of the Faraday rotator 113 in the direction perpendicular to the growing face of the film is at most 400 μm or so, and the obtainable Faraday rotation angle could be 45° or so. Accordingly, the polarization controller that requires a variable polarization rotation angle range of at least 180 degrees must use multiple (in general, 6 to 8) Faraday rotators 113. This is further problematic in that the size of the magneto-optical device of the type is difficult to reduce and the power to be consumed by structure is also difficult to reduce.
Patent Document 1: Japanese Patent No. 2,815,509
Patent Document 2: U.S. Pat. No. 5,657,151
Patent Document 3: JP-A 7-199137
Patent Document 4: U.S. Pat. No. 4,239,337
Patent Document 5: U.S. Pat. No. 3,420,601
Non-Patent Document 1: Kazuhiro Ikeda and 5 others, Endless Tracking Polarization Controller, Furukawa review, January 2003, No. 23, pp. 32–38